This invention relates generally to the field of microchemical sensors, and more particularly to using electrical parameters to differentiate changes in surface mass from changes in chemical solution properties using a quartz crystal microbalance.
The quartz crystal microbalance (QCM) is commonly configured with electrodes on both sides of a thin disk of AT-cut quartz. Because of the piezoelectric properties and crystalline orientation of the quartz, the application of a voltage between these electrodes results in a shear deformation of the crystal. The crystal can be electrically excited into resonance when the excitation frequency is such that the crystal thickness is an odd multiple of half the acoustic wavelength. At these frequencies, a standing shear wave is generated across the thickness of the plate for the fundamental and higher harmonic resonances.
QCMs were originally used in vacuo to measure deposition rates. As shown by Sauerbrey, Z. PHYS., Vol. 155, pp. 206-222 (1959), changes in the resonant frequency are simply related to mass accumulated on the crystal, and this teaching has been implemented in U.S. Pat. No. 4,788,466, entitled "Piezoelectric Sensor Q-loss Compensation," to Paul et al., Nov. 29, 1988; and U.S. Pat. No. 4,561,286, entitled "Piezoelectric Contamination Detector," to Sekler et al., Dec. 31, 1985; and U.S. Pat. No. 4,391,338, entitled "Microbalance and Method for Measuring the Mass of Matter Suspended Within a Fluid Medium," to Patashnick et al., Jul. 5, 1983. Lu et al., J. APPL. PHYS.Vol. 43, pp. 4385-4390 (1972) showed that the QCM typically may be used as an instrument in the frequency-control element of an oscillator circuit; a precise microbalance is realized by monitoring changes in oscillation frequency. More recently, QCMs have been shown to operate in contact with fluids by Numura et al., NIPPON KAGAKU KAISHI, pp. 1121(1980) enabling their use as solution phase microbalances. This microbalance capability has facilitated a number of solution measurements, as in, for instance, U.S. Pat. No. 4,741,200, entitled "Method and Apparatus for Measuring Viscosity in a Liquid Utilizing a Piezoelectric Sensor," to Hammerle, May 3, 1988. Other examples include deposition monitoring as taught by Schumacher, ANGEW. CHEM. INT. ED. ENGL., Vol. 29, pp. 329-343 (1990), and U.S. Pat. No. 4,311,725, entitled "Control of Deposition of Thin Films," by Holland, Jan. 19, 1982; species detection by Deakin et al., ANAL. CHEM., Vol. 61, pp. 290-295 (1989); immunoassay by Thompson et al., ANAL. CHEM. Vol. 58, pp. 1206-1209 (1986), and U.S. Pat. No. 4,999,284, entitled "Enzymatically amplified piezoelectric specific binding assay," to Ward et al., Mar. 12, 1991; fluid chromatographic detection shown by Konash et al., ANAL. CHEM., Vol. 52, pp. 1929-1931 (1980); corrosion monitoring by Seo et al., EXTENDED ABSTRACTS--178TH MEETING OF THE ELECTROCHEMICAL SOCIETY, Abstract No. 187, p. 272, Seattle, WA (1990); and electrochemical analysis taught by Bruckenstein et al., J. ELECTROANAL. CHEM., Vol. 188, pp. 131-136 (1988), and by Ward et al., SCIENCE, Vol. 249, pp. 1000-1007 (1990).
Kanazawa et al., ANAL. CHEM., Vol. 57, pp. 1770-1771 (1985), have shown that QCMs operating in solution are also sensitive to the viscosity and density of the contacting solution. Viscous coupling of the fluid medium to the oscillating device surface results in both a decrease in the resonant frequency of the QCM and damping of the resonance.
But, prior to the invention described herein, no one has suggested that the mass and the liquid properties can be measured simultaneously. In fact, no one thought that the measurement of mass loading onto a QCM would be affected by the changes in the density and viscosity of the liquid. Indeed, the prior art assumed the fluid density to be constant. But, because the resonant frequency is affected by both mass and fluid loading, measurement of the resonant frequency alone cannot distinguish changes in surface mass from changes in solution properties. Those of the prior art who measured the viscosity or the density of the fluid simply did not have the means to measure the solid mass accumulation. In fact, their measurement of the fluid properties would be in error if there were any solid mass accumulation on the surface of the QCM. And, those in the prior art measuring solid mass accumulation had to assume that the fluid properties remained constant, otherwise their measurement of mass would be in error. But, in fact, the change in frequency is dependent upon both the mass and the fluid properties, and by measuring specific electrical characteristics over a range of frequencies near resonance, the QCM can differentiate between these loading mechanisms.
It is thus an object of the invention to present a QCM simultaneously loaded by a thin surface mass layer and a viscous fluid. The invention takes advantage of the derived analytical expression for QCM admittance as a function of excitation frequency.
Accordingly, the invention is a method to determine total mass of a solid and/or physical properties of a fluid, both the mass and fluid contacting the same quartz crystal microbalance, comprising applying an oscillating electric field across the thickness of the quartz crystal microbalance in contact with a solid mass interposed between the quartz crystal microbalance and a fluid, then measuring at least one resonant frequency of the quartz crystal microbalance, simultaneously measuring the admittance magnitude at the resonant frequencies, and correlating the resonant frequency and the admittance magnitude to obtain a surface mass density and a fluid viscosity-density product. A second embodiment of the invention comprises applying an oscillating electric field across the thickness of a quartz crystal microbalance, sweeping a frequency over a range that spans at least one resonant frequency of the crystal, measuring the magnitude and phase of the admittance over the frequency range, correlating the admittance data to the frequency, and applying the admittance/frequency correlation to an equivalent circuit model, contacting a solid mass and/or a fluid onto the crystal wherein the solid mass is interposed between the crystal and the fluid, repeating the steps sweeping the frequency range that spans a resonant frequency, measuring the magnitude and phase of the admittance over that frequency range, and correlating the admittance data to the frequency and then applying the admittance/frequency correlation to an equivalent circuit model, and then extracting the solid mass and fluid density-viscosity product from the correlated admittance/frequency data.